Readability Scale
The readability scale is the standard method for rating the intelligibility of a radio signal. Both ATC and pilots use it for sender-receiver communication.
Source: ICAO Annex 10 Volume II, Chapter 5.2.2.5; ICAO Doc 9432.
The 5-step scale
| Step | Meaning | Description |
|---|---|---|
| 1 | Unreadable | unreadable — no comprehension possible |
| 2 | Readable now and then | occasionally readable — mostly unintelligible |
| 3 | Readable but with difficulty | readable with effort — words need multiple hearings |
| 4 | Readable | readable — standard |
| 5 | Perfectly readable | perfectly readable — like telephone |
Use
Pilot asks (typical scenario after frequency change or unclear receive):
- "Munich Tower, DEMRA, how do you read?"
ATC replies:
- "DEMRA, readability 5" (perfect)
- "DEMRA, readability 3" (with difficulty)
- "DEMRA, broken transmission" (very poor)
Conversely ATC asks:
- "DEMRA, how do you read?"
Pilot replies:
- "Munich Tower, readability 5, DEMRA."
Standard formulations
English
- Readability 5 = perfect.
- Readability 4 = readable.
- Readability 3 = with difficulty.
- Readability 2 = occasionally.
- Readability 1 = unreadable.
Typical causes of low readability
- Transmitter fault: weak signal strength.
- Antenna issue: broken, poorly grounded.
- Microphone position wrong: too far, too close.
- Background noise: loud cockpit, wind.
- Weak reception: too far from ground station.
- Interference from other transmitters or electronics.
Measures for low readability
Reception improvement
- Adjust squelch (radio knob).
- Climb higher (improve LOS range).
- Check antenna (visible damage).
Transmission improvement
- Microphone closer to mouth.
- Speak louder — but don't shout.
- Articulate more clearly.
- Reduce breathing between words.
Frequency change
- For chronically poor receive quality: try another frequency (if available, e.g. alternate FIS).
Special cases
"Broken transmission"
- Several short statements with comprehension issues → ATC requests repetition.
"Read you loud and clear"
- Informal phrasing for "readability 5", in military or older civil context.
"Strength" as alternative
Some sources use a "strength + readability" scale (S5/R5), from maritime / amateur radio tradition. In modern ICAO procedures only readability is used.
Example sentence in flight
Pilot: "Munich Approach, DEMRA, level 5000, how do you read?"
ATC: "DEMRA, Munich Approach, readability 5, continue inbound."
→ Pilot knows: reception is perfect, continue as planned.
Summary
The readability scale is part of professional radio discipline. It helps to communicate communication quality objectively rather than judgementally ("you're hard to understand") but technically ("readability 3").